Flame retardant



United States 3,371,058 FLAME RETARDANT Carl F. Perizzolo, Richmond,Calif, assignor to Stautfer Chemicai Company, New York, N.Y., acorporation of Delaware No Drawing. Filed May 27, 1965, Ser. No. 459,4177 Claims. (U. 260-291) ABSTRACT (BF THE DESCLGSURE This inventionrelates to a durable flame retarding composition for wood. Morespecifically, the present invention is directed to providing an improvedan effectively durable useful flame retardant to 'be used in conjunctionwith wood in order to impart thereto effective flame retardingcharacteristics.

Wood is very stable at ordinary temperatures. However, at elevatedtemperatures wood undergoes degradation and combustion. The degradationof wood at elevated temperatures is a process of decomposition of solidcomponents thereof into volatile liquids and gases and is accompanied bya reduction in weight. Wood becomes more porous, less dense and moreeasily ignited as it undergoes thermal decomposition. Therefore, firewhen controlled is an aid to mankind but when uncontrolled it is thenatural enemy of wood. It is generally accepted that it is impossible torender Wood indestructible by fire, but the prior art contains manytreatments that will reduce or prevent flame propagation. Sometreatments are more or less effective. The thermal decomposition of woodcan be affected catalytically by the presence of some inorganicchemicals, such as, for example, compounds containing iron, copper andchromium ions. The more important consideration is the use of additivechemicals to produce a fire-retarding effect on wood. Some examples ofknown fire-retardant chemicals are ammonium phosphate, ammonium sulfate,ammonium chloride, zinc chloride, antimony chloride mixtures of boraxand boric acid as well as various organic chemicals. These chemicals donot inhibit thermal decomposition of wood at elevated temperatures, butthey do inhibit flaming combustion or glowing of the wood. Varioustheories have been presented on the flame retarding action of variousknown additives. The observation that added chemicals can change thedirection of wood pyrolysis leads those in the field to search for newand effective fire-retarding treatment systems. The general nature ofthe fire-retardant chemicals impart to the wood resistance to ignition,to fire penetration and to the spread of flame over the wood surface.

In addition to flame retarding the wood, it is desirable to employ atreatment which is also durable after being exposed to rigorousweathering conditions. The treatment is to be durable in the sense thatsuflicient flame retarding material remains in the wood upon repeatedexposure to water, heat and ultraviolet light to continue tosatisfactorily flame retard the wood after a given length of time. Manysystems have been proposed to accomplish this end but each has failed inone regard or another. Some treatments are prohibitive because ofeconomics, destructive degradation of the wood cell structure, i.e.,

- atent 3,371,658 Patented Feb. 27, 1968 loss of material strength,leachable by-products, and the like.

A principal object of this invention is to provide an effective anddurable flame retarding treatment for wood, wherein wood treatedtherewith will display fire retarding characteristics. Another object isprovision of a process for carrying out said treatment on wood. Morevor,the invention affords development of an effective and durable fireretarding composition easily applicable to wood to render said wood lesssusceptible to destruction by fire.

Pursuant to the above-mentioned and yet further objects, it has beenfound surprisingly that wood treated with phosphorus oxychloride impartsto said treated wood certain desirable and durable flame-retardingproperties. It has also been found that these effective and durableflame retarding properties of phosphorus oxychloride in wood may beextended favorably by the incorporation of a suitable binding resintherewith. Such a supplemental treatment protects the primary flameretarding agent, phosphorus oxychloride, against excessive leaching bywater. However, it is not necessary to employ a binder resin herein aswill become apparent from the remainder of the specification. Excellentdurability results may be obtained from the use of phosphorusoxychloride alone.

There have been several processes which employ phosphorus oxychloride toimpart flame retarding properties to essentially pure cellulosicmaterial. However, wood as used in the present invention may be definedas an interpenetrating system of high polymers. The major componentpolymers comprising said system may be classified as cellulose,approximately one-half the wood weight, and lignin, approximately twentyto thirty percent of the wood tissue. Other components present in minoramounts in wood may be terpenes, proteins, aliphatic acids, sterols andvarious inorganic constituents. It should be noted in the description ofwood that its composition is markedly nonuniform. Therefore, wooddiffers from pure cellulose in that it has present in addition tocellulose various other natural polymeric materials. In the prior artthat describes the phosphorylation of cellulose utilizing phosphorusoxychloride, it was found necessary to employ an organic base such aspyridine, ammonia, ethylene diamine, urea and the like in order toobtain retention of the necessary phosphorus content. Within theembodiments of the present invention it is possible to achieve effectiveand durable fire retardant properties in wood by employing onlyphosphorus oxychloride or phosphorus oxychloride and a binder resin.

In the treating solution used in the practice of the present inventionan unreactive organic solvent is employed as a carrier for thephosphorus oxychloride and binding resin, if one is used. The use of asolvent aids in the penetration of the wood structure. The applicationof various concentrations of flame retardant in a suitable solvent isalso a convenient means for control of the load desired in the Wood. Anyinert essentially anhydrous solvent such as kerosene, naphtha, benzene,hexane, butane, chlorine-containing solvents or other inert liquid whichdoes not react with the fire retarding composition utilizing phosphorusoxychloride, may be employed. Carbon tetrachloride, methylene chloride,chloroform and the like all may serve as acceptable solvents. Sometimesit may be found advantageous to include in the treating solution afungicide and/or bactericide to retard fermentation and the growth offungi harmful to wood.

Employment of phosphorus oxychloride with wood in quantities whichconfer a desired beneficial flame retarding effect is dependent upon theeffect desired. Whereas very low loadings of phosphorus oxychloride areoperable as flame-retardants, the specific amount to provide a desiredperformance is readily determinable by those skilled in the art simplyby visual observation or by use of conventional techniques. Hence, inorder to arrive at an optimum beneficial effect suited to the purposefor which the wood is selected, only routine testing involving variationof the loading levels of phosphorus oxychloride is generally required.

In the treating composition an associative adjuvant may be used incooperation with phosphorus oxychloride, wherein lseser amounts ofphosphorus oxychloride are needed to impart effective and durable flameretarding characteristics to the treated wood. Said adjuvant may be anysuitable binder resin possessing waterproofing characteristics, such aspolyvinyl chloride, polyvinylidene chloride, vinyl chloride-vinylacetate copolymer, vinyl chloride-vinylidene chloride copolyrner,halogenated polyesters, halogenated epoxy resins, polystyrene,polyethylene, polyb-utenes, polymethylmethacrylate, aminealdehyderesins, epoxy resins, unsaturated polyesters, and the like. Thepreferred binding resin is vinyl chloride-vinyl acetate copolymer. Saidresin may be any mixture of the two compounds which, afterpolymerization by any known or suitable method, such as heat or theaction of ultraviolet light radiation, or by the action of a substancelike benzoyl peroxide, may be taken up in a convenient organic solventfor application to the wood. The inventor does not Wish to be limited toa specific resin composi tion, since any composition of copolymer may befound applicable.

Two general methods are available for applying flame retarding materialsto wood. One is an application of coatings of non-combustible materialsover the surface of the Wood. The other is an impregnation process whichdeposits the chemicals within the wood structure. The coatings can beoil, resin or latex base preparations in which the fire-retardingchemical has been incorporated. The coatings may be of the intumescenttype, that is, those which froth and swell at flame temperatures. Theimpregnation processes usually involve injecting chemicals in solutioninto the Wood by pressure techniques in order to obtain predeterminedabsorptions. The compositions herein described may be incorporated intoeither of the application methods, although the preferred process isthat of pressure impregnation.

In carrying out the preferred method of treating wood with the fireretarding compositions of the present invention, two general proceduresfor pressure impregnation of wood may be used. These will be referred tohereinafter as the empty cell and the full cell procedures,respectively.

In the empty cell procedure the wood is placed in a pressure: vessel,commonly a horizontal cylinder, and moderate air pressure is applied. Animpregnant solution is then forced into the cylinder and into the wood.Pressures of up to 100-150 pounds per square inch are used to force theimpregnant solution deep into the wood. The pressure is then releasedand the unabsorbed solution is removed from the cylinder. A vacuum isthen applied and the residual air in the capillaries of the woodexpands, thereby forcing the excess liquid into the pores of the wood.The wood pores are in this way covered with a thin film of theimpregnant.

In the full cell procedure a high vacuum is applied initially to thepressure vessel containing the wood. An impregnating solution is addedto the cylinder and surrounds the wood while the vacuum is maintained.Positive pressure is then applied to force the impregnant into the wood.At the end of the impregnation the pressure is released. A final vacuumis applied and the unabsorbed solution is removedfrom the cylinder.Since the air in the pores of the wood Was removed from the pores by theinitial vacuum, a considerable amount of liquid now remains in thepores. However, the relatively small amount of air which was not removedfrom the pores by the initial vacuum dissolved in the solution duringthe pressure period. When the pressure is released, this dissolved aircomes out of solution in the form of bubbles, causing the woodto weep.The application of the finalvacuum prevents or minimizes weeping byrapidly removing the solution that would normally run out by the actionof the expansion of the dissolved air.

If time is no object, it is unnecessary to use pressure and diffusionalone may be relied upon. Over a period of time Wood immersed in an opentank of solution will have the 7 air in the pores displaced by theliquid. I

It must be recognized that the flame retarding composi* tion containingphosphorus oxychloride and a binding resin may be applied to the wood ina two-stage, as well as a one-stage, combined-component system. In thenormal practice of a two-stage treatment system any of theaforedescribed procedures are equally applicable, as well ascombinations of the procedures. For example, the empty cell techniquefor introduction of phosphorus oxchloride followed by a full cellprocedure for the binder resin. Various other modifications incombination or separately may be made Within the pressure impregnatingprocesses described above. For example, the full cell procedure may bemodified to exclude the initial evacuation step. Likewise, the finalevacuation step may be eliminated and the treated wood dried as soon asthe pressure is released and the excess impregnant solution is removed.

Since the quantity of the flame retarding agent of the present inventionwhich is employed with wood will vary with the fire-retarding effectsought, the utility of the wood, i.e., exterior or interior application,and the type and dimensions of the wood treated, it is evident that norigid load limits can be set forth. For many purposes, particularly inlarge volume wood members, very low quantities on a weight to weightbasis are satisfactory. Obviously deep penetration of the flameretardant composition into the interior of a large volume wood member isnot necessary for satisfactory performance, since only the exteriorportion and a limited depth thereof are susceptible to degradation andcombustion. For other purposes, wherein the wood member has a rathersmall volume, loading levels may reach a high ratio of flame retardingagent to wood on a weight to weight basis. At the same time requirementsof durability must be determined. Exterior exposure to severe weatheringconditions will require higher load limits as compared to an interiorutility. Determination of the optimum quantity for a specific flameretarding performance is readily conducted by routine procedures, aswill be apparent to those skilled in the art.

An accelerated exposure test was employed in the evaluation ofphosphorus oxychloride as a durable flame retarding agent for woodwithin the embodiments of the present invention. It is known that mostof the inorganic and organic fire-retardant chemicals described in theprior art also cause discoloration of the treated wood, rusting or otheradverse deterioration of metal fastening means, are toxic to humans, orare easily removed by leaching, as by rain, from the wood thereby nolonger effective as a fire-retardant. In order to shorten the time ofexposure tests to study the present system, the use of artificialenvironments was adapted. Three durability tests were employed: analternate rain and heat test wherein the samples were exposed to 3.5days of rain equivalent to 12.5 inches per hour and 3.5 days of sun at65 C. The rain exposure can be equated to the severe conditions of 525inches of rain per year for approximately 24 years. This test wasconducted for a total of up to 12 weeks; a heat test wherein the sampleswere kept at C. for days, and a weatherometer test consisting of 600hours of exposure to noon sunlight, cooling, heating and moisture. Theweatherometer test was run according to American Society for TestingMaterials procedure type D test 1342-57.

The herein described treatment for wood with phosphorus oxychloride orphosphorus oxychloride and a binder resin does not display saidundesirable characteristics as described, supra, before or after thedurability tests. Therefore, wood treated with an application ofphosphorus oxychloride and a binding resin, if one is employed, improvesthe resistance of the wood to ignition, to fire penetration and to thespread of flame over the wood surface without aflecting the appearanceof the wood, the metal fastening used therewith, endangering humans byexposure thereto, or rapid removal by adverse and severe weatheringconditions.

The applicant does not fully understand the mechanism involved in theincorporation of the phosphorus oxychloride in wood and therefore doesnot wish to be limited to any specific mode of incorporation. However,it is thought that there is a reaction between phosphorus oxychlorideand various constituents that go to make up the complex structure ofwood. This is evidenced by the fact'that the system of the presentinvention is operable as a durable permanent flame retarding agent forwood. Theoretically, a large number of possible chemical structuralformulas can be drawn for reaction products of phosphorus oxychlorideand various constituents of wood and it is highly probable that thereaction products in wood are mixtures of several different of thesecompounds having diiferent structural formulas. Many of the previouslyproposed flame retardant systems are easily leached from wood, therebysuggesting non-reaction with the wood, but a loose absorptiveincorporation. Therefore, these prior systems are non-durable toweathering conditions and the like.

The following examples illustrate the present invention as applied towood.

Example 1 This example illustrates the impregnation of a selectedrepresentative wood specie with a loading of phosphorus oxychloridesufficient to be effective both before and after exposure to severeweathering conditions.

Samples of red cedar, 4 in. x 6 in. by A in. were accurately weighed andused as the test specimen. The pieces of wood were placed in a pressurevessel. The impregnation technique used was a modified full cellprocedure. The chamber was evacuated to a vacuum of about 15 to 20inches for approximately 30 minutes. A solution of 4 percent phosphorusoxychloride in methylene chloride as solvent was introduced into thepressure vessel sufficient to fill the pressure chamber containing thewood specimen. Hydrostatic pressure of approximately 90 pounds persquare inch was then applied to the liquid in the pressure vessel untilthe wood samples no longer absorbed the liquid. The pressure was thenreleased and the unabsorbed solution was removed from the chamber. Theentire process was carried out at room temperature. The wood specimenswere dried to constant weight at 65 C. in an oven. The load ofphosphorus oxychloride absorbed was equivalent to approximately 7.43pounds per 100 pounds of wood, after drying. Analysis indicated 1.50percent phosphorus present in the wood.

Prior to testing the durability of the system in wood the wood sampleswere exposed to a gas/air flame 1% inches by 6 /2 inches ofapproximately 1300 F. An external air supply equivalent to a 12mile-perhour Wind was superimposed on the test flame and wood samples.The sample pieces of wood were arranged in a 3-member deck simulating aroof structure. Wood samples were exposed for four minutes and for threecycles comprising 1 minute flame exposure and 2 minutes rest. Once thesamples passed these flame exposure tests satisfactorily they weresubjected to the following durability tests.

Various sets of wood samples thus treated were subjected to accelerateddurability tests before exposure to flame. The rain/heat test consistedof alternate exposure over a 12 week period to 3.5 days of rain at 12.5inches/hour and 3.5 days of heat at 65 C. The severe heat test consistedof 90 days exposure to a temperature of 80 C. The weatherorneter testconsisted of a 600-hour exposure to alternating cycles of 18 minutes ofspray and 102 minutes of arc. The temperature during the arcing cyclewas 145 F. The wood samples subjected to the durability tests were thenexposed to the above-described intermittent gas/air flame test.

Observations were made such as the spread of the flame beyond area offlame impingement, afterglow on removal of the flame (structuralstrength of the specimen, and the like. There was essentially no flamespread, no afterglow or at best several small glow areas which wereshort-lived and the specimen did not disintegrate or becomesubstantially weakened after flame removal. The wood samples treatedwith phosphorus oxychloride as described did exhibit flame retardantcharacteristics in all phases of the testing procedure.

Example 2 This example illustrates a typical loading of phosphorusoxychloride sufficient for most interior, non-weathering applications.

In the same manner as Example 1, samples of red cedar, 4 in. x 6 in. x 7in., were accurately weighed and used as the test specimen. Using themodified full cell procedure the wood samples were impregnated with aloading of phosphorus oxychloride equivalent to approximately 1.5 poundsper pounds of wood. Analysis yielded 0.3 percent phosphorus.

These samples were not subjected to the durability tests of Example 1.However, they were exposed to the gas/ air flame at a temperature ofapproximately 1300 F. with the superimposed air source equivalent to a13 mileper-hour wind. The wood samples were arranged in a 3- mernberdeck as previously described and exposed for four minutes to the testflame and also for three cycles of the alternate exposurerest flametest.

The wood samples thus treated and exposed to the test flame hadexcellent flame retardant characteristics. These characteristics wereexemplified by no flame spread beyond the area of flame impingement,retained structural strength of the specimen and little or no afterglowon removal of the flame.

Example 3 This example illustrates the use of a typical binder resin inconjunction with a lower loading requirement of phosphorus oxychloridesufiicient to be durable and present excellent flame retardantcharacteristics.

In the same manner as Example 1, samples of red cedar, 4 in. x 6 in. x91 in., were accurately weighed and used as representative testspecimen. Using the modified full cell procedure the wood samples wereimpregnated with a loading of phosphorus oxychloride and vinylchloride-vinyl acetate copolymer as the binder resin. The impregnantsolution was 2% phosphorus oxychloride, 5% copolymer resin (13% vinylacetate-87% vinyl chloride) and 93% methylene chloride. The loadingobtained was equivalent to 6.7 pounds of phosphorus oxychloride andcopolymer resin per 100 pounds of wood. This is equivalent to 1.13pounds of phosphorus oxychloride, 0.23% phosphorus. Similarly, woodsamples were impregnated with a solution consisting of 4% phosphorusoxychloride, 10% copolymer resin (defined, supra), and 86% methylenechloride. The loading obtained was equivalent to 8 pounds of phosphorusoxychloride and copolymer resin per 100 pounds of wood. This isequivalent to 1.58 pounds of phosphorus oxychloride, 0.32% phosphorus.Both sets of samples were dried to constant weight.

The wood samples thus treated were arranged in the usual 3-membered decksimulating a roof structure and subjected initially to the gas/air testflame for four minutes. No adverse burning or other degradative effectswere observed.

Sample sets were now subjected to the previously described acceleratedweathering conditions. After nine weeks in the rain-heat testthe samplesof lower loading, i.e., 1.12 pounds of phosphorus oxychloride per 100pounds of wood, satisfactorily resisted the three cycle 1- 7minute-Z-minute flame-rest exposure test. The sample set with 1.58pounds of phosphorus oxychloride per 100 pounds of wood satisfactorilypassed the three cycle 1- minute-2-minute flame-rest exposure test aftertwelve weeks of subjection to the rain/heat test.

From these tests it can be seen that the degree of permanency of theflame retardant effect of the compositions of the present invention onwood is dependent upon the amount of the phosphorus compound retained bythe wood after exposure to the severe accelerated weathering conditions.Therefore the degree of permanency or durability required is a functionof the flame retarding performance desired under given conditions. Ascan be seen from the above examples, lower phosphorus oxychloride loadsmay be suflicient for an interior non-weather exposed application. Atthe same time the lower loading may be combined with a suitable binderresin or waterproofing composition which diminishes leaching of thephosphorus compound by water and thereby become adequately satisfactoryfor various exterior weatherable applications.

Wood samples treated in accordance with the present invention were foundto retain their original natural wood color, exhibit no evidence ofdegradation, exhibit no excessive corrosion of metal fastening meanseither before or after weathering tests, retain their structuralstrength, no warping was evident because of the treatment and the woodsamples were not hygroscopic beyond normal moisture absorption from theatmosphere.

Various changes and modifications may be made without departing from thespirit and scope of the invention described herein as will be apparentto those skilled in the art to which it pertains. It is accordinglyintended that the present invention shall only be limited by the scopeof the appended claims.

I claim:

1. An efiective flame retardant composition for wood consisting ofphosphorus oxychloride, vinyl chloride-vinyl acetate copolymer, and aninert organic solvent therefor.

2. A process for the preparation of flame retardant wood consisting ofapplying to said wood an effectiveflame-retarding amount of phosphorusoxychloride.

3. A process for the preparation of flame retardant wood consisting ofapplying to said rwood an effective flame-retarding amount of a solutionof phosphorus oxychloride and a binder resin possessing waterproofingcharacteristics.

4. A process for the preparation of flame retardant Wood consisting ofapplying to said wood an eflective flame-retarding amount of a solutionof phosphorus oxychloride and vinyl chloride-vinyl acetate copolymer.

5. A process for the preparation of flame retardant wood consisting ofimpregnating said wood with an effective flame-retarding amount ofphosphorus oxychloride.

6'. A process for the preparation of flame retardant Wood consisting ofimpregnating said wood with an effective flame-retarding amount ofphosphorus oxychloride and a binder resin possessing waterproofingcharacteristics.

7. A process for the preparation of flame retardant wood consisting ofimpregnating said wood with an effective flame-rearding amount ofphosphorus oxychloride and vinyl chloride-vinyl acetate copolymer.

References Cited UNITED STATES PATENTS 2,680,077 3/1954 Nielsen et all17-l37 X 3,067,173 12/1962 Barney 260-45.7 X 3,081,293 3/1963 Doughty106-15 X OTHER REFERENCES 7 Dufrais'se et al.: Co'mpt. rend 192, 564-6(1931).

DONALD E. CZAJA, Primary Examiner. M. I. WELSH, Assistant Examiner

